The present invention is suited for use in a storage apparatus with a virtual/logical volume to which a dynamically variable storage area is allocated, the volume being provided to a host computer.
In recent years, storage apparatuses providing a host computer with storage areas for storing data have been able to include quite a large number of large-capacity disk drives, and the storage apparatus capacity has been expanding. This type of storage apparatus is operated so that: a disk array configured based on RAID (Redundant Array of Independent Disks) is generated from several disk devices; a plurality of so generated physical storage resources is then collected to make a physical/logical volume; and a storage area of a capacity needed by a host computer is taken out from that physical/logical volume to make a logical volume to be provided to the host computer.
Furthermore, another type of storage apparatus has recently been proposed where, instead of making a logical volume of a fixed capacity from a physical/logical volume, a host computer is initially provided with a virtually defined logical volume (hereinafter referred to as a virtual/logical volume), and, in response to the host computer's command, a dynamically variable storage area is allocated from a physical/logical volume (i.e., a physical resource) in particular units to that virtual/logical volume, thereby the storage capacity being dynamically expanded.
For example, JP Patent Laid-Open Publication No. 2003-015915 discloses a storage apparatus that provides each host computer with a corresponding virtual/logical volume made of a plurality of disk memory devices; obtains the read/write target logical block address from a command from the host computer directed to the virtual/logical volume; and if the virtual/logical volume has no storage area associated with the logical block address that has been specified by the command, allocates a storage area from unused magnetic disk memory devices so that the storage area for the virtual/logical volume is dynamically expanded.
However, the above storage apparatus disclosed by JP Patent Laid-Open Publication No. 2003-015915 is configured to allocate a storage area to the virtual/logical volume in predetermined fixed units of allocation. So, if the fixed allocation unit size for the storage area for storing data sent from the host computer is large, that large portion of the storage area will be allocated even when a small piece of data is sent from the host computer, resulting in lower storage area operation efficiency.
On the other hand, in the storage apparatus disclosed by JP Patent Laid-Open Publication No. 2003-015915, the fixed allocation unit size for the storage area for storing data sent from the host computer is small, it is necessary to increase the number of management bits for managing the storage area allocated, and huge memory capacity is required to maintain those management bits.